Re: [Bier] BIER: draft-eckert-bier-cgm2-rbs-01 with performance analysis simulation

Jeff,

It is indeed initially somewhat counterintuitive to think that the CGM2/RBS 
bitstring encoding could be more efficient (less copies) than BIER - given how BIER
does not even need to encode the tree but just the leaves. Whereas CGM2/RBS does
encode the tree.

The reason becomes IMHO more obvious, when one considers that BIER needs to send a
separate packet copy even if just ONE BFER in the bitstring needs a copy. If we have
a BSL of 256 bits, it means we encoded 255 "dead-weight" bits. And if we want to
replicate a packet to N% out of a large number of BFER one can imagine how we 
get to the points where where the bitstring of every packet copy will have only a
few bits set.

With the compressed tree model, the length of each bitstring of interest is just
the total number of direct adjacencies (BFER or BFR), so the efficiency of each
such bitstring is never as low as 1/256, but maybe just 1/20 - if the BFR has
20 neighbors (other BFR or BFER). So within a max of 256 bits of "variable length
encoded tree", there is maybe better than 10% of bits to which a copy is made.

Meaning: I wouldn't want to bet a large amount of money on how exactly 
the comparison would play out when we increase the BSL size for both BIER and
CGM2/RBS (as you suggested), because then the efficiency of a BSL=512 bitstring
in BIER could be as low as 1/512 if only one bit is set there. Whereas the
percentace "replication efficiency per bit of addressing" in CGM2/RBS would probably
stay the same.

Cheers
    Toerless

On Wed, Feb 09, 2022 at 08:49:26PM +0000, Jeffrey (Zhaohui) Zhang wrote:
> Hi Toerless,
> 
> Not sure if my understanding is correct, but it seems that RBS does not reduce the number of bits that are needed to encode the tree. Rather, it increases the number of bits (to encode the recursive structure).
> I agree that it reduces the size of BIFTs, but even current BIER-TE can reduce the number of copies if you use a longer bitstring?
> 
> Jeffrey
> 
> 
> Juniper Business Use Only
> 
> -----Original Message-----
> From: BIER <bier-bounces@ietf.org> On Behalf Of tte@cs.fau.de
> Sent: Wednesday, February 9, 2022 2:19 PM
> To: bier@ietf.org
> Cc: bing.xu@huawei.com
> Subject: [Bier] BIER: draft-eckert-bier-cgm2-rbs-01 with performance analysis simulation
> 
> [External Email. Be cautious of content]
> 
> 
> Dear BIER-TE WG:
> 
> Robin did add a section (6.3) describing an initial performance gain analysis of CGM2/RBS to the github source (https://urldefense.com/v3/__https://github.com/toerless/bier-cgm2-rbs__;!!NEt6yMaO-gk!WS49OT72vlonSWP3yLtLcW_RQARYP00KEiAWpH592AuDXmrOOJH_bgVzQZyfK9En$ ), and i just did a bit of editorial fixup and posted it as -01 of the draft.
> 
> This actually is the first time i actually like the HTML'ized version of a draft, because the topology picture is so large it doesn't fit a single page:
> 
> https://urldefense.com/v3/__https://www.ietf.org/archive/id/draft-eckert-bier-cgm2-rbs-01.html__;!!NEt6yMaO-gk!WS49OT72vlonSWP3yLtLcW_RQARYP00KEiAWpH592AuDXmrOOJH_bgVzQaBIgBPC$
> 
> The interesting piece about the comparison is that it is actually comparing CGM2/RBS to BIER, and not BIER-TE. Because BIER itself should be requiring less copies than BIER-TE, so the gain of CGM2/RBS over BIER-TE should be even higher, but the fact alone that you get away with fewer packet copies to large receiver sets even though the bitstring also needs to encode the path/tree towards the receivers is really cool.
> 
> Robin, two Q:
> 
> 1. The new text mentions "in our graphs", but the text does not include any such graphs (yet).
> I guess such a graph would be even worse to convert to ASCII than the topology.
> Maybe post whatever format you have those results in to github (PDF, png...) and then we actually may want to see if/how a PDF version of the draft could include better than just ASCII art. Certainly a good reason to finally try it out.
> And short term we can just add references to such visuals to the draft.
> 
> 2; Is it correct to assume that the hops through the topology that you simulated are "just" shortest-path, maybe with some ECMP choice - aka: the same paths that also BIER would choose given some "default" IGP routing setup ?
> 
> Cheers
>     Toerless
> 
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